1. Field of the Invention
The invention relates generally to the field of weed management. More specifically, the invention relates to methods for using auxin-like herbicides such as dicamba for controlling weeds.
2. Description of the Related Art
Weeds cost farmers billions of dollars annually in crop losses and the expense of efforts to keep weeds under control. Weeds also serve as hosts for crop diseases and insect pests. The losses caused by weeds in agricultural production environments include decreases in crop yield, reduced crop quality, increased irrigation costs, increased harvesting costs, decreased land value, injury to livestock, and crop damage from insects and diseases harbored by the weeds. The principal means by which weeds cause these effects are: 1) competing with crop plants for the essentials of growth and development, 2) production of toxic or irritant chemicals that cause human or animal health problem, 3) production of immense quantities of seed or vegetative reproductive parts or both that contaminate agricultural products and perpetuate the species in agricultural lands, and 4) production on agricultural and nonagricultural lands of vast amounts of vegetation that must be disposed of. The damage caused can be significant. For example, it is estimated that between 1972 and 1976 corn yields were reduced by about 10% due to weeds (Chandler, 1981).
Among weeds that serve as hosts for crop pests, for example, pepperweed and tansymustard (Descurainia sp.) maintain large populations of diamondback moths during the late fall, winter, and spring. They are also hosts to the turnip aphid and green peach aphid. Several weed species of the nightshade family (Solanaceae) are hosts to insects that commonly attack eggplant, pepper, potato, and tomato. For example, horsenettle (Solanum carolinense L.) is a host of the Colorado potato beetle, and black nightshade (S. nigrum L.) is a host of the cabbage looper. Morning-glory is an important host of insects attacking sweet potato, especially the highly destructive sweet potato weevil. Ragweed serves as a host for Mansonia mosquitoes, an insect vector for the human diseases encephalitis and rural filariasis.
Some weeds are undesirable in hay, pastures, and range-lands because of the mechanical injury that they inflict on livestock. Woody stems, thorns, and stiff seed awns cause injury to the mouth and digestive tract of livestock; and the hairs and fibers of some plants tend to ball up and obstruct the intestines, especially in horses, causing serious problems. Ingested by milk cows, some weeds such as ragweeds, wild garlic (Allium vineale L.), and mustard, among others, impart a distinctly distasteful odor or flavor to milk and butter. Barbed seed dispersal units may become so entangled in the wool of sheep as to greatly diminish its market value. Parasitic plants, such as dodder (Cuscuta sp.), broomrape (Orobanche sp.), and witchweed, rob their host plants of organic foodstuffs.
Chemical herbicides have provided an effective method of weed control over the years. Herbicides can generally be applied pre-emergence and/or post-emergence. Pre-emergence herbicides are applied in a field before a crop emerges from the soil. Such applications are typically applied to the soil before, at the same time, or soon after planting the crop. Such applications may kill weeds that are growing in the field prior to the emergence of the crop, and may also prevent or reduce germination of weeds that are present in the soil. Post-emergence herbicides are typically used to kill weeds after a crop has emerged in the field. Such applications may kill weeds in the field and prevent or reduce future weed germination. In either case, the herbicides may be applied to the surface of the soil, mixed with the soil, over the top of the plant, or applied by any other method known to those of skill in the art.
One weed control strategy is to apply an herbicide such as dicamba to a field before sowing seeds. However, after applying the herbicide to a field, a farmer has to wait at least several weeks before sowing the field with crop seeds such that the herbicide has killed most of the weeds and has degraded so as not injure the sown crop. For example, plants are especially sensitive to dicamba and it has been recommended that dicamba formulations such as Banvel™ or Sterling™ be applied 30 days prior to planting for controlling weeds. A comprehensive list of weeds that are controlled by dicamba is available (Anonymous, 2007). The herbicide is particularly useful for control of taller weeds and more difficult to control weeds such as purslane, sicklepod, morninglory and wild buckwheat. Dicamba can be used to control weeds not susceptible to other herbicides. Following the application of Clarity™, another formulation of dicamba, a minimum accumulation of one inch of rainfall or overhead irrigation followed by a 14 day waiting period for the 4 to 8 ounce/acre rates or a 28 day waiting period for the 16 ounce/acre rates has been recommend for controlling weeds in a soybean field (see Table 22 in VanGessel and Majek, 2005). Also, the Clarity® label recommends that it be applied at least 15 days prior to sorghum planting. Similarly, for cotton, a waiting period of 21 days is recommended after applying Clarity® or Banvel® to the field, before planting the cotton seeds (Craig et al., 2005, Crop Profile for Cotton (Gossypium hirsutum) in Tennessee, www.ipmcenters.org/cropprofiles/docs/tncotton.html) and no pre-emergence and post-emergence application are recommended. The waiting period is also dependent on the crop growing environment at any give time, such as the type of soil (soil having organic activity will degrade dicamba faster), moisture content, rainfall, temperature, as well as type of formulation and rate of application.
The herbicide 2,4-D has been recommended for controlling certain weeds in a soybean field such as mustard spp., plantains, marestail, and 2,4-D susceptible annual broadleaf weeds by applying it 7 to 30 days prior to planting, depending on rate and formulation (ester or amine) (see Table 22 in VanGessel and Majek, 2005).
One method that has been successfully used to manage weeds combines herbicide treatments with crops that are tolerant to the herbicides. In this manner, herbicides that would normally injure a crop can be applied before and during growth of the crop without causing damage. Thus, weeds may be effectively controlled and new weed control options are made available to the grower. In recent years, crops tolerant to several herbicides have been developed. For example, crops tolerant to 2,4-dichlorophenoxyacetic acid (Streber and Willmitzer, 1989), bromoxynil (Stalker et al., 1988), glyphosate (Comai et al., 1985) and phosphinothricin (De Block et al., 1987) have been developed.
Recently, a gene for dicamba monooxygenase (DMO) was isolated from Pseudomonas maltophilia (US Patent Application No: 20030135879) which is involved in the conversion of a herbicidal form of the herbicide dicamba (3,6-dichloro-o-anisic acid) to a non-toxic 3,6-dichlorosalicylic acid. The inventors reported the transformation of the DMO gene into tobacco and Arabidopsis. The transformed plant tissue was selected on kanamycin and regenerated into a plant. However, herbicide tolerance was not demonstrated or suggested in immature tissues or seedlings or in other plants. Pre-emergence herbicide tolerance to dicamba was not described. Transgenic soybean plants and other plants tolerant to application of dicamba are described in Behrens et al. (2007).
Dicamba is one member of a class of herbicides commonly referred to as “auxin-like” herbicides or “synthetic auxins.” These herbicides mimic or act like the natural plant growth regulators called auxins. Auxin-like herbicides appear to affect cell wall plasticity and nucleic acid metabolism, which can lead to uncontrolled cell division and growth. The injury symptoms caused by auxin-like herbicides include epinastic bending and twisting of stems and petioles, leaf cupping and curling, and abnormal leaf shape and venation.
Dicamba is one of the many auxin-like herbicides that is a low-cost, environmentally-friendly herbicide that has been used as a pre-emergence herbicide (i.e., 30 days prior to planting) in dicots and as a pre- and/or post-emergence herbicide in corn, sorghum, small grains, pasture, hay, rangeland, sugarcane, asparagus, turf, and grass seed crops to effectively control annual and perennial broadleaf weeds and several grassy weeds (Crop Protection Chemicals Reference, 1995). Unfortunately, dicamba can injure many commercial crops including beans, soybeans, cotton, peas, potatoes, sunflowers, tomatoes, tobacco, and fruit trees, ornamental plants and trees, and other broadleaf plants when it comes into contact with them. Soybean and cotton are particularly sensitive to dicamba. Thus, applications of dicamba must generally occur several weeks before planting of sensitive crops to ensure that residual dicamba is sufficiently cleared from the crop environment before crops emerge. For post-emergent weed control in corn, dicamba is the 5th most widely used herbicide for broad leaf weeds. However, although the optimal rate for broad leaf weed control is between 280 to 560 g/h (grams/hectare), the average use rate in corn is 168 g/h as at higher use rates and under certain environmental conditions, dicamba can injure corn.
As noted above, current manufacturer's guidelines typically require at least a 30 day delay between the application of dicamba and the planting of sensitive crops. This inability to apply dicamba close to the time that crops are planted delays sowing time and shortens the growing season, thereby increasing the risk of exposing crops to frost in the fall. The delay also means that the farmers have to go through the field twice; once for planting and once for spraying, thereby increasing fuel and wear-tear costs to the farmers. Improvements over the state of the art that would eliminate the delay would positively impact the quality and quantity of the crop which could result and reduce economic losses to farmers. More effective weed control would also reduce the risk of weeds developing resistance to existing herbicides.